ENH: Switch to TOML and use Pydantic for validation #1401

Job	Run time
docs	44s
test (ubuntu-latest, 3.8, release)	11m 49s
test_environments (ubuntu-latest, release)	2m 21s
test (ubuntu-latest, 3.12, release)	12m 6s
test_environments (macos-latest, release)	2m 36s
test (ubuntu-latest, pypy-3.10, release)	14m 20s
test (macos-13, 3.8, release)	25m 45s
test (macos-13, 3.12, release)	28m 14s
test (macos-13, pypy-3.10, release)	24m 28s
	2h 2m 23s

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